Allosteric control is the mechanism whereby a control molecule binds to a site on a protein, inducing a conformational change at a distant site, which affects the function of the protein. It is a fundamental molecular control mechanism in the cell: enzymes are typically allosterically controlled (e.g. hexokinase); gene expression is regulated locally by allosteric control of repressors (e.g. the Tryptophan repressor), and non-locally through looping induced by DNA-binding proteins. The latter is a simple example of how binding at one site can effectively modify the chemistry at a distant site: the essence of allosteric control.
An allosterically controlled enzyme is a chemical amplifier: it takes one molecule to switch the enzyme on, but many molecules are synthesized as products of the catalytic reaction. Building artificial molecular devices with similar “chemical transistor” properties has evident scientific and technological interest by creating amplified molecular probes.